Fast and Effective Embedded Systems Design. Applying the ARM mbed

366 Pages·2012·10.61 MB·English

by  Rob Toulson

CHAPTER 1
Embedded Systems, Microcontrollers
and ARM
Chapter Outline
1.1 Introducing Embedded Systems 3
1.1.1 What isan EmbeddedSystem? 3
1.1.2 AnExample Embedded System 5
1.2 Microprocessors and Microcontrollers 6
1.2.1 SomeComputer Essentials 7
1.2.2 TheMicrocontroller 8
1.3 Development Processes in Embedded Systems 9
1.3.1 Programming Languages: Whatis soSpecial aboutC/Cþþ? 9
1.3.2 TheDevelopment Cycle 10
1.4 The World of ARM 11
1.4.1 ALittleHistory 12
1.4.2 SomeTechnical Detail: What doesthis RISCWord Mean? 13
1.4.3 TheCortex Core 14
Chapter Review 15
Quiz 15
References 16
1.1 Introducing Embedded Systems
1.1.1 What is an Embedded System?
Weareallfamiliarwiththeideaofadesktoporlaptopcomputer,andtheamazingprocessing
thattheycando.Thesecomputersaregeneralpurpose;wecangetthemtododifferentthings
atdifferenttimes,dependingontheapplicationorprogramwerunonthem.Attheveryheart
ofsuchcomputersisamicroprocessor,atinyandfantasticallycomplicatedelectroniccircuit
which contains the core features of a computer. All of this is fabricated on a single slice of
silicon, called an integrated circuit (IC). Some people, particularly those who are not
engineers themselves, call these circuits microchips, or just chips.
FastandEffectiveEmbeddedSystemsDesign.DOI:10.1016/B978-0-08-097768-3.00001-5
Copyright(cid:1)2012ElsevierLtd.Allrightsreserved. 3
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What is less familiar to many people is the idea that instead of putting a microprocessor in
a general-purpose computer, it can also be placed inside a product which has nothing to do
with computing, like a washing machine, toaster or camera. The microprocessor is then
customizedtocontrolthatproduct.Thecomputeristhere,insidetheproduct;butitcannotbe
seen,and theuserprobablydoesnoteven know itis there.Moreover,thoseadd-onsthatare
normallyassociatedwithacomputer,likekeyboard,screenormouse,arenowheretobeseen.
Such products are called embedded systems, because the computer that controls them is
embedded right inside. Because they tend to focus on control, in many cases the
microprocessors used in embedded systems develop different characteristics from the ones
used in more general-purpose machines. They are called microcontrollers. Although much
lessvisiblethantheirmicroprocessorcousins,microcontrollerssellinfargreatervolumeand
their impact has been enormous. To the electronic and system designer they offer huge
opportunities.
Embedded systems come in many forms and guises. They are extremely common in the
home,themotorvehicleandtheworkplace.Mostmoderndomesticappliances,likewashing
machines, dishwashers, ovens, central heating and burglar alarms, are embedded systems.
Themotorcarisfullofthem,inenginemanagement,security(forexample,lockingandanti-
theftdevices),air-conditioning,brakes,radio,andsoon.Theyarefoundacrossindustryand
commerce,inmachinecontrol,factoryautomation,robotics,electroniccommerceandoffice
equipment. The list has almost no end, and it continues to grow.
Figure1.1expressestheembeddedsystemasasimpleblockdiagram.Thereisasetofinputs
from the controlled system. The embedded computer, usually a microcontroller, runs
(User Interface)
TIME
Embedded Computer
Input Software Output
Variables Variables
HHaarrddwwaarree
(Link to other Systems)
Figure 1.1:
The embedded system
Embedded Systems, Microcontrollers and ARM 5
a program dedicated to this application, permanently stored in its memory. Unlike the
general-purpose desktop computer, which runs many programs, this is the only program it
ever runs. Based on information supplied from the inputs, the microcontroller computes
certain outputs, which are connected to things like actuators within the system. The actual
electronic circuit, along with any electromechanical components, is often called the
hardware;theprogramrunningonitisoftencalledthesoftware.Asidefromallofthis,there
may also be interaction with a user, for example via keypad and display, and there may be
interaction with other subsystems elsewhere, although neither of these is essential to the
generalconcept.Oneothervariablewillaffectallthatwedoinembeddedsystems,andthisis
time,representedasadominatingarrowwhichcutsacrossthefigure.Wewillneedtobeable
tomeasuretime,makethingshappenatpreciselypredeterminedtimes,generatedatastreams
orothersignalswithastrongtimedependence,andrespondtounexpectedthingsinatimely
fashion.
This chapter introduces or reviews many concepts relating to computers, microprocessors,
microcontrollersandembeddedsystems.Itdoesthisinoverviewform,togiveaplatformfor
further learning. We return to most concepts in later chapters, building on them and adding
detail. More details can also be found in Reference 1.1.
1.1.2 An Example Embedded System
A snack vending machine is a good example of an embedded system. One is represented in
Figure 1.2, in block diagram form. At its heart is a single microcontroller. As the diagram
shows, this accepts a number of input signals, from the user keypad, the coin-counting
module and the dispensingmechanism itself. It generates output signals dependent on those
inputs.
A hungrycustomermay approachthe machine and start jabbingat thebuttons or feedingin
coins. In the first case, the keypad sends signals back to the microcontroller, so that it can
recognizeindividualkeyspressedandthenhookthesekeyvaluestogethertodecipheramore
complex message. The coin-counting module will also send information on the amount of
money paid. The microcontroller will attempt to make deductions from the information it
receives and will outputstatus informationon the liquid crystal display. Has avalid product
been selected? Has enough money been paid? If yes, then it will energize an actuator to
dispensetheproduct.Ifno,itwilldisplayamessageaskingformoremoneyorare-entryof
theproductcode.Agoodmachinewillbereadytogivechange.Ifitdoesdispense,therewill
besensorsonthemechanismtoensurethattheproductisavailable,andfinallythattheaction
is complete. These are shown in the diagram as gate position sensors. A bad machine (and
haven’t we all met these?) will give annoying or useless messages on its display, demand
moremoneywhenyouknowyou’vegiven therightamount,or leave thatchocolatebar you
desperately want teetering on a knife-edge, not dropping down into the dispensing tray.
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Figure 1.2:
Vending machine embedded system
What has been described so far is a fairly conventional machine. We can, however, take it
further. In modernvending systems there might be a mobile communications feature which
allowsthevendingmachinetoreportdirectlytothemaintenanceteamifafaultisdiagnosed;
theywillthencomeandfixthefault.Similarly,themachinemightreportstocklevelsthrough
mobileorInternetcommunicationstoallowtheserviceteamtovisitandreplenishwhenever
supplies run low.
This simpleexamplereflectsexactly thediagramofFigure1.1.Themicrocontrolleraccepts
inputvariables,makescalculationsanddecisionswiththis,andgeneratesoutputsinresponse.
Itdoesthisinatimelymanner,andcorrectuseoftimeisimplicitintheseactions.Thereisin
this case a user interface, and in a modern machine there is a network interface. While the
above paragraphs seem to be describing the physical hardware of the system, in fact it is all
controlledbythesoftwarethatthedesignerhaswritten.Thisrunsinthemicrocontrollerand
determines what the system actually does.
1.2 Microprocessors and Microcontrollers
Letusgobacktothemicrocontroller,whichsitsattheheartofanyembeddedsystem.Asthe
microcontrollerisinessenceatypeofcomputer,itwillbeusefulforustogetagrasponbasic
computerdetails.Wedothisincompleteoverviewhere,butreturntosomeofthesefeatures
in later chapters.
Embedded Systems, Microcontrollers and ARM 7
1.2.1 Some Computer Essentials
Figure 1.3 shows the essential elements of any computer system. As its very purpose for
existence, a computer can perform arithmetic or logical calculations. It does this in a digital
electronic circuit called the arithmetic logic unit (ALU). The ALU is placed within a larger
circuit, called the central processing unit (CPU), which provides some of the supporting
features that it needs. The ALU can undertake a number of simple arithmetic and logic
calculations. Which one it does depends on a code which is fed to it, called an instruction.
Nowwearegettingclosertowhatacomputerisallabout.IfwecankeeptheALUbusy,by
feeding it a sensible sequence of instructions, and also pass it the data it needs to work on,
then we have the makings of a very useful machine indeed.
The ability to keep feeding the ALU with instructions and data is provided by the control
circuit which sits around it. It is worth noting that any one of these instructions performs
a very simple function. However, because the typical computer runs so incredibly fast, the
overall effect is one of very great computational power. The series of instructions is called
aprogram,normallyheldinanareaofmemorycalledprogrammemory.Thismemoryneeds
tobepermanent.Ifitis,thentheprogramisretainedindefinitely,whetherpowerisappliedor
not, and it is ready to run as soon as power is applied. Memory like this, which keeps its
contentswhenpowerisremoved,iscallednon-volatilememory.Theold-fashionednamefor
thisisreadonlymemory(ROM).Thislatterterminologyisstillsometimesused,eventhough
it is no longer accurate for new memory technology. The control circuit needs to keep
accessingtheprogrammemory,tofindoutwhatthenextinstructionis.ThedatathattheALU
works on may be drawn from the data memory, with the result placed there after the
calculationiscomplete.Usuallythisistemporarydata.Thismemorytypethereforeneednot
bepermanent,althoughthereisnoharmifitis.Memorythatlosesitscontentswhenpoweris
removed is called volatile memory. The old-fashioned name for this type of memory is
data and
address
Data
Central Memory
data and Processing
The Input/ address Unit (CPU)
Outside
Output
World
data and
address
Program
ALU
Memory
Control
Figure 1.3:
Essentials of a computer
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random access memory (RAM). This terminology is still used, although it conveys little
useful information.
To be of any use, the computer must be able to communicatewith the outside world, and it
does this through its input/output (I/O). On a personal computer this implies human
interaction, through things like the keyboard, visual display unit (VDU) and printer. In an
embeddedsystem,atleastasimpleone,thecommunicationislikelytobeprimarilywiththe
physical world around it, through sensors and actuators. Data coming in from the outside
world might be quickly transferred to the ALU for processing, or it might be stored in data
memory. Data being sent out to the outside world is likely to be the result of a recent
calculation in the ALU.
Finally, there must be data paths between each of these main blocks, as shown by the block
arrows in the diagram. These are collections of wires, which carry digital information in
eitherdirection.Onesetofwirescarriesthedataitself,forexamplefromprogrammemoryto
theCPU;thisiscalledthedatabus.Theothersetofwirescarriesaddressinformationandis
calledtheaddressbus.Theaddressisadigitalnumberthatindicateswhichplaceinmemory
thedatashouldbestoredin,orretrievedfrom.Thewiresineachofthedataandaddressbuses
could be tracks on a printed circuit board or interconnections within an IC.
One of the defining features of any computer is the size of its ALU. Simple, old processors
were 8bit, and some of that size still have useful roles to play. This means that, with their
8bits, they can represent a number between 0 and 255. (Check Appendix A if you are
unfamiliarwithbinarynumbers.)Morerecentmachinesare32or64bit.Thisgivesthemfar
greater processing power, but adds to their complexity. Given an ALU size, it generally
followsthatmanyotherfeaturestakethesamesize,forexamplememorylocations,databus,
and so on.
Asalreadysuggested,theCPUhasaninstructionset,whichisasetofbinarycodesthatitcan
recognize and respond to. For example, certain instructions will require it to add or subtract
twonumbers,orstoreanumberinmemory.Manyinstructionsmustalsobeaccompaniedby
data,oraddressestodata,onwhichtheinstructioncanoperate.Fundamentally,theprogram
that the computer holds in its program memory and to which it responds is a list of
instructionstakenfromtheinstructionset,withanyaccompanyingdataoraddressesthatare
needed.Suchcodeissometimescalledmachinecode,todistinguishitfromotherversionsof
the program that we may also develop.
1.2.2 The Microcontroller
A microcontroller takes the essential features of a computer as just described, and adds to
thesethefeaturesthatareneededforittoperformitscontrolfunctions.Itisusefultothinkof
it as being made up of three parts e core, memory and peripherals e as shown in the block
Embedded Systems, Microcontrollers and ARM 9
Program Further
Memory Peripheral
Power
Data Further
Memory Peripheral
Digital
I/0
Microprocessor
Core
Reset
Analog
Interrupt(s)
I/0
Internal Data &
Address Buses
Counters
Clock
& Timers
Figure 1.4:
Features of an example microcontroller: core þ memory þ peripherals
diagram of Figure 1.4. The core is the CPU and its control circuitry. Alongside this go the
program and data memory. Finally, there are the peripherals. These are the elements that
distinguishamicrocontrollerfromamicroprocessor,fortheyaretheelementsthatallowthe
wide-ranging interaction with the outside world that the microcontroller needs. Peripherals
canincludedigitaloranaloginput/output,serialports,timers,countersandmanyotheruseful
subsystems.
It almost goes without saying, but must not be forgotten, that the microcontroller needs
power,intheformofastabledirectcurrent(DC)supply.Afurtherrequirement,aswithany
personal computer (PC), is to have a clock signal. This is the signal that endlessly steps the
microcontrollercircuitthroughthesequenceofactionsthatitsprogramdictates.Theclockis
often derived from a quartz oscillator, such as you may have in your wristwatch. This gives
a stable and reliable clock frequency. The clock often has a very important secondary
function, of providing essential timing information for the activities of the microcontroller,
for example in timing the events it initiates, or in controlling the timing of serial data.
1.3 Development Processes in Embedded Systems
þþ
1.3.1 Programming Languages: What is so Special about C/C ?
The CPUinstructionsetwas mentionedin Section1.2.1.Asprogrammers,it isour ultimate
goal to produce a program that makes the microcontroller do what we want it to do; that
program must be a listing of instructions, in binary, drawn from the instruction set. The
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programinthisrawbinaryformiswhatwehavealreadycalledmachinecode.Itisextremely
tedious, to the point of being near impossible, for us as humans to work with the binary
numbers that form the instruction set.
AfirststeptowardssanityinprogrammingiscalledAssembler.InAssemblereachinstruction
gets its own mnemonic, a little word which a human can remember and work with. The
instruction set is represented by a set of mnemonics. The program is then written in these
mnemonics,andacomputerprogramcalledacross-assemblerconvertsallthosemnemonics
into the actual binary code that is loaded into program memory. Assembler has its uses, for
one it allows us to work very closely with the CPU capabilities; Assembler programs can
thereforebeveryfastandefficient.YetitiseasytomakemistakesinAssemblerandhardto
find them, and the programming process is time consuming.
A further step to programming sanity is to use a high-level language (HLL). In this casewe
use a language like C, Java or Fortran to write the program, following the rules of that
language.Thenacomputerprogramcalledacompilerreadstheprogramwehavewritten,and
converts(compiles)thatintoalistingofinstructionsfromtheinstructionset.Thisassumeswe
have made no mistakes in writing the program! This list of instructions becomes the binary
code that we can download to the program memory. Our needs in the embedded world,
however,arenotlikethoseofotherprogrammers.Wewanttobeabletocontrolthehardware
we are working with, and write programs that execute quickly, in predictable times. Not all
HLLs are equally good at meeting these requirements. People get very excited, and debate
endlessly, about what is the best language in the embedded environment. However, most
agreethat,formanyapplications,Chasclearadvantages.Thisisbecauseitissimpleandhas
featuresthatallowustogetatthehardwarewhenrequired.AstepupfromCisCþþ,which
is also widely used for more advanced embedded applications.
1.3.2 The Development Cycle
Thisbookisaboutdevelopingembeddedsystems,anddevelopingthemquicklyandreliably,
so it is worth getting an early picture of what that development process is all about.
In the early days of embedded systems, the microcontrollers were very simple, without
on-chip memory and with few peripherals. It took a lot of effort just to design and build the
hardware. Moreover, memory was very limited, so programs had to be short. The main
development effort was spent on hardware design, and programming consisted of writing
rathersimpleprogramsinAssembler.Overtheyears,themicrocontrollersbecamemoreand
more sophisticated, and memory much more plentiful. By now, many suppliers were selling
predesignedcircuitboardscontainingthemicrocontrollerandallassociatedcircuitry.Where
these were used there was very little development effort needed for the hardware. Now
attention could be turned to writing complex and sophisticated programs, using all the
Embedded Systems, Microcontrollers and ARM 11
Write/Revise Source Code
Assemble/Compile
(Simulate)
Download
Test in Hardware
Figure 1.5:
The embedded program development cycle
memory that had become available. This tends to be the situation we find ourselves in
nowadays.
Despitethesechanges,theprogramdevelopmentcycleisstillbasedaroundthesimpleloop
showninFigure1.5.Inthisbook,wewillmainlywritesourcecodeusingC.Thediagramof
Figure1.5impliessomeoftheequipmentwearelikelytoneed.Thesourcecodewillneedtobe
writtenonacomputerelet’scallitthehostcomputereforexampleaPC,usingsomesortof
texteditor.Thesourcecodewillneedtobeconvertedintothebinarymachinecodewhichis
downloadedtothemicrocontroller,placedwithinthecircuitorsystemthatitwillcontrol(let’s
callthisthetargetsystem).Thatconversionisdonebythecompiler,runningonthehost
computer.Programdownloadtothetargetsystemrequirestemporaryconnectiontothehost
computer.Thereisconsiderableclevernessinhowtheprogramdataisactuallywrittenintothe
programmemory,butthatneednotconcernushere.Beforetheprogramisdownloaded,itis
oftenpossibletosimulateitonthehostcomputer;thisallowsaprogramtobedevelopedto
agoodlevelbeforegoingtothetroubleofdownloading.However,thetruetestoftheprogram
istoseeitrunningcorrectlyinthetargetsystem.Whenprogramerrorsarefoundatthisstage,
astheyinevitablyare,theprogramcanberewritten,andcompiledanddownloadedoncemore.
1.4 The World of ARM
The development of computers and microprocessors has at different times been driven
forward by giant corporations or by tiny start-ups; but always it has been driven forward by
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verytalentedindividualsorteams.ThedevelopmentofARMhasseenacombinationofallof
these. The full history of the development of ARM is an absolutely fascinating one, like so
manyotherhi-techstart-upsinthepast30orsoyears.Atinysummaryisgivenbelow;doread
it in a fuller version in one of the several websites devoted to this topic. Then watch that
history continue to unfold in the years to come!
1.4.1 A Little History
In 1981 the British Broadcasting Corporation launched a computer education project and
askedforcompaniestobidtosupplyacomputerthatcouldbeusedforthis.Thewinnerwas
the Acorncomputer.This becameanextremely popularmachine,and wasvery widelyused
in schools and universities in the UK. The Acorn used a 6502 microprocessor, made by
a company called MOS Technology. This was a little 8-bit processor, which would not be
takenvery seriously these days, but was respected in its time. Responding to the growing
interestinpersonalordesk-topcomputers,IBMin1981produceditsveryfirstPC,basedon
a more powerful Intel 16-bit microprocessor, the 8088. There were many companies
producing similar computers at the time, including of course Apple. These early machines
were prettymuchincompatiblewitheach other,anditwasquiteunclearwhetheronewould
finallydominate.Throughoutthe1980s,however,theinfluenceoftheIBMPCgrew,andits
smallercompetitorsbegantofade.DespiteAcorn’sUKsuccessitdidnotexportwell,andits
future no longer looked bright.
It was around this time that those clever designers at Acorn made three intellectual leaps.
They wanted to launch a new computer. This would inevitably mean moving on from the
6502, but they just could not find a suitable processor for the sort of upgrade they needed.
Their first leap was the realization that they had the capability to design the microprocessor
itself and did not need to buy it in from elsewhere. Being a small team and experiencing
intense commercial pressure, they designed a small processor, but one with real
sophistication. The computer they built with this, the Archimedes, was a very advanced
machine, but struggled against the commercial might of IBM. The company found
themselveslookingatcomputersaleswhichjustwerenotsufficient,butholdingthedesignof
anextremelyclevermicroprocessor.Theyrealizedetheirsecondleapethattheirfuturemay
not lie in selling the completed computer itself. Therefore, in 1990, Acorn computers
cofounded another Cambridge company, called Advanced RISC Machines Ltd, ARM for
short. They also began to realize e their third leap e that you do not need to manufacture
silicontobeasuccessfuldesigner,whatmatteredwastheideasinsidethedesign.Thesecan
be sold as intellectual property (IP).
The ARM concept continued to prosper, with a sequence of smart microprocessor designs
being sold as IP to an increasing number of major manufacturers around the world. The
company has enjoyed huge success, and is currently called ARM Holdings. Thosewho buy